This invention relates to dynamoelectric machines, and methods of their construction, that include premanent magnets assembled with laminated pole members.
Available permanent magnets, such as rare earth magnets, have made it attractive to construct dynamoelectric machine rotors with permanent magnets rather than wound coils for providing field excitation. Key advantages of the permanent magnet rotors are reductions in weight and size which are particularly important in applications such as aircraft.
Aircraft generators have traditionally been AC synchronous machines usually with a brushless excitation system on the rotor involving a rotating rectifier construction with a field winding fed by a separate exciter generator. Constructing the rotor of permanent magnets eliminates the exciter generator, the field winding, and related rotating rectifier. This need for a small auxiliary permanent magnet generator to supply start-up power is also eliminated. Aircraft generators are normally driven at a constant speed by means of a hydraulic speed conversion system operating off of a variable speed engine. Now it has become desirable to operate aircraft generators at a variable speed, eliminating the hydraulic speed conversion system, and to develop a constant frequency electrical output by electronic power conversion systems such as a cycloconverter system. Such a power conversion system can also permit operation of the generator in a reverse mode to obtain motor operation useful for starter-generator systems on aircraft engines or flywheel energy storage and retrieval. A brushless wound field machine is not as attractive for this purpose because it requires an exciter generator whose size has to be increased for capability of operating at decreased speed.
Another area of interest for application of machines with permanent magnet rotors is in brushless DC motors. These offer advantages in efficiency and in size and weight as opposed to wound rotors. The opportunity is available to obtain low rotor inertia, using rare earth magnets, in actuators for control surfaces in aircraft or other applications where quick response is required. Electric rather than hydraulic actuators are therefore made possible and provide an opportunity for totally electric aircraft systems.
One construction for a permanent magnet rotor consists of the magnets having their magnetic axes radially disposed with the magnet surface exposed to the machine air gap. The magnets are anchored, such as by an adhesive bonding material, to an underlying ferrous member which conducts magnetic flux from pole to pole. While this is a relatively simple construction, it is not suitable for high speed operation because of the weak structure of the magnets themselves and the joints at which they are bonded to the ferrous member. Another disadvantage, when rare earth magnets are used, is the low flux density in the air gap resulting from the normal characteristics of rare earth magnets which have a relatively low flux density-high coercive force characteristic. Therefore, this type of construction does not use the magnets effectively. Another disadvantage, with magnets that are electrically conductive, results from the stator slot openings which cause a high frequency variation in the magnetic flux on the surface of the magnet as it rotates resulting in high eddy current losses.
To obtain a higher strength structure, the preceding described rotor can be modified to contain the magnets inside a high strength enclosure. Such an enclosure normally requires ferrous material radially above the magnets to conduct the flux to the air gap and non-ferrous material between the magnets to avoid magnetically shorting them. This type of structure still does not use rare earth magnets effectively, has high eddy current losses in the solid enclosure and, also, is difficult to manufacture because it requires weldments between ferrous and non-ferrous materials.
A form of high strength enclosure construction which attempts to address some of these problems is shown in FIG. 1. It is described in Technical Report AFAPL-TR-76-8, March 1976, "150 KVA Samarium Cobalt USCF Starter Generator Electrical System, Phase 1 , " by General Electric Company, Aircraft Equipment Div., Binghamton, New York, for Air Force Aeropropulsion Laboratory, Air Force Wright Aeronautical Laboratories, Air Force Systems Command, Wright Patterson Air Force Base, Ohio 45433. Here, elongated, rectangular cross-section, magnets 10 are disposed to run in radial planes through the rotor core. The intervening core material is of magnetic steel laminations or lamination segments 12. The periphery of the rotor comprises non-ferrous metal strips 14 running over the face of the permanent magnets. The strips 14 are welded together with partial cylindrical ferrous steel members 16 over the pole region. The generated magnetic flux paths 18 result from an orientation of the magnetic axis of the magnets circumferentially in the rotor. Thus, the flux path is out of one side of a magnet 10 into the ferrous pole piece adjacent to it and then turns radially into the machine air gap. The flux returns from the air gap through an adjacent pole down into the other side of the magnet. Because the area of the magnet can be extended radially, the flux from one magnet can be compressed into a relatively small area of the ferrous material in the pole at the air gap. This utilizes the magnet material much more effectively than the structure previously referred to. However, the disadvantages of high eddy current losses and difficulty of manufacture remain.
Various aspects of permanent magnet rotor construction in accordance with the prior art, including some features as discussed above as well as others, and various aspects of permanent magnet rotor application are described in the following representative patents: Harley U.S. Pat. No. 2,059,518, Nov. 3, 1936; Yates U.S. Pat. No. 3,492,520, Jan. 27, 1970; Knudson et al. U.S. Pat. No. 3,671,788, June 20, 1972; Richter U.S. Pat. No. 4,117,360, Sept. 26, 1978; and Steen U.S. Pat. No. 4,139,790, Feb. 13, 1979.
The present invention achieves the multiple objectives of providing a permanent magnet rotor in a high strength construction, with low eddy current losses, effective utilization of permanent magnets, and complete containment of the magnets in an economically manufactured structure. Briefly, the rotor structure comprises a core of a plurality of stacked laminations that include predominantly magnetic laminations that have interspersed among them non-magnetic laminations for structural strength. The magnets are located in radial planes running longitudinally through the core and have a circumferential magnetic field orientation for effective magnet usage. The magnets are secured by wedges extending longitudinally through the core at the outer periphery with the wedges being of non-magnetic material that is mechanically locked in place within laminations while the outer edges of the laminations between wedges form pole members and are exposed to the air gap. A plurality of bead welds extend longitudinally over the surface of the core to secure the laminations into a unit while minimizing the surface area of the core that is shielded from the air gap. In a preferred form of the invention, the bead welds securing the rotor laminations are coordinated with the stator slot pattern such that they are spaced from each other by a distance equivalent to the stator slot pitch to minimize current flow from induced voltages in the welds.
In accordance with the method of assembly of this invention, a stack of laminations is assembled on an arbor using slots on the outside diameter of the high strength lamination with wedge-shaped locating fixtures that extend into the slots to locate and hold the magnetic laminations preparatory to welding. The bead welds are then applied to run across the length of the stack. The permanent magnets are then inserted in the slots in the stack, after welding the laminations, so as to avoid any necessary machining operation of the laminations with a magnetized magnet in place. Then nonferrous metal or high strength non-metallic wedges are inserted into the outer ends of the slots to retain the magnets without requiring welding operations. Mechanical strength is further improved by dipping the assembly in a varnish dip and then drying it.